It's a mixture of things I think. He had an apple pie life where he could put hunting on the back burner and live happily with Lisa and Ben, then hunting returns with a seriously screwed up brother and other surprises. Earlier he was just a hunter and I think he used his cocky attitude to make what was essentially his career more bearable. Now that he sees hunting differently as just something he's doing before he can get back to Lisa and Ben, I think the cockiness is just a waste of time in his perspective. That's a part of it I think, then there's Sam that is cocky and tempermental now and Dean sees what its like dealing with a smartass that has to be babysat is like. Maybe it puts things in perspective to what Sam went through for five seasons and Dean just doesn't have the time or energy to be positive.

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